This invention relates to a tri-state driver arrangement in accordance with the pre-characterising portion of claim 1, and in particular a tri-state driver arrangement, which can be used to send data from a subscriber via a communications bus to a further subscriber using an interface, for example in accordance with the RS-485 standard.
The RS-485 standard is used to define the transmission of information across different communications busses. Here, the communications bus consists of a two-wire cable to which the individual bus subscribers are connected in parallel and where several bus subscribers are able to receive data sent from another bus subscriber at the same time (so-called xe2x80x9cmulti-dropxe2x80x9d configuration). On account of the high number of node points and the need for a reliable transmission of data or signals, all bus subscribers are potentially isolated from the communications bus. Furthermore, the line drivers or output drivers associated with the individual bus subscribers must have tri-state characteristics, i.e. along with the states xe2x80x9cLxe2x80x9d and xe2x80x9cHxe2x80x9d, they can take a third, high-impedance state. If a bus subscriber is active, i.e. if the output driver associated with this bus subscriber is sending data, all the other bus subscribers must be in the receive mode and disconnect their output drivers from the communications bus, i.e. switch to the above-mentioned tri-state condition.
FIG. 5 shows an example of a potentially isolated RS-485 interface according to the state of the art. The RS-485 interface incorporates a tri-state driver 12 acting as a transmitter with an output driver 16, which is connected to a two-wire bus-cable A, B. Transmitted data TxD are fed to the tri-state driver 12 and amplified by means of an amplifier 14. A fast optocoupler 15 transfers the transmitted data with potential isolation to the output driver 16. Furthermore, an activation or enable signal is fed to the tri-state driver 12 and amplified by means of an amplifier 17. A slow-acting optocoupler transfers the enable signal to the output driver 16 in the form of an appropriate voltage UEN1 via a further amplifier 19 with potential isolation, whereby the output driver 16 is activated or deactivated depending upon the value of the voltage UEN1, i.e. switched to the tri-state condition. Furthermore, the RS-485 interface incorporates a receiver 13, which is also connected to the two-wire bus-cable A, B, whereby an amplifier 20 receives and amplifies data transmitted from another subscriber, which are output as received data RxD via a further fast optocoupler 21 and a driver 22.
As can be seen from FIG. 5, the RS-485 interface or the appropriate tri-state driver 12 needs two channels, each with an optocoupler, for sending the transmitted data TxD and for activating/deactivating the output driver 16, i.e. the number of components required and the driver power are relatively high.
This invention is therefore based on producing a tri-state driver arrangement, which enables the potentially isolated transmission of signals with minimum driver outlay.
This problem is solved according to the invention by a tri-state driver arrangement with the characteristics of claim 1 or 11. The sub-claims each define preferred and advantageous embodiments of this invention.
According to one embodiment, the tri-state driver arrangement according to the invention uses a magnetic coupler for the potentially isolated transmission of signals between the input and the output of the tri-state driver arrangement, which can be formed in particular due to a sensor, which works on the principle of the so-called GMR (Giant Magnet Resistor) effect. Likewise, however, other types of magnetic coupler can also be used, such as for example, Hall sensors, signal transformers, etc.
Due to the use of the magnetic coupler, only one channel is required for transmitting information or data and for the activation/deactivation of the output driver of the tri-state driver arrangement. Compared with the state of the art described above, this means a saving on components and driver power. Furthermore, the use of the magnetic coupler allows a saving to be made on the area of silicon required as well as the bonding effort when the tri-state driver arrangement according to the invention is used in a potentially isolated RS-485 interface and this potentially isolated RS-485 interface is integrated into a housing.
Furthermore, a suitable logic or control is proposed within the framework of this invention, which evaluates the output signal from the coupler, in particular the output voltage from the coupler, with respect to a pre-defined limiting range and, depending on this, produces an activation or enable signal for the output driver in order to activate or deactivate this for transmitting data, i.e. to switch it to the high-impedance tri-state condition. In doing so, it is particularly advantageous if the enable signal mentioned is fed to the output driver via a smoothing filter with an appropriate time constant so that the output driver is not unnecessarily switched off for short intervals of time. The procedure suggested within the framework of this invention for evaluating the output signal of the coupler in order to activate the output driver for transmitting data or to switch it to the tri-state condition and thus deactivate it dependent upon this, can be achieved particularly easily and without great expense and, in principle, is independent of the type of coupler used in each case, as long as this has a sufficiently linear output characteristic, which makes it possible to evaluate the output signal of the coupler in accordance with the invention.
This invention is particularly suitable for use in potentially isolated RS-485 interfaces. Of course however, this invention can also be used wherever it is required to transmit potentially isolated signals or data between an input and an output using a tri-state characteristic.
This invention is described in more detail below in the light of preferred embodiments with reference to the attached drawing.